Dust Suppressants

The research of chemical stabilizers on dust suppression begun in the 1930s with about more than 150 kinds of chemical stabilizers invented to date and counting in about 40 different countries. Categorically, chemical stabilizers can be divided into traditional and non-traditional types. Where traditional dust suppressants are to be applied in road environments, there are numerous dictating factors that must first be considered. 

These include the following:

• Road surface moisture content during application;
• Hydrological conditions such as precipitation and appropriate drainage;
• Mechanical stability of the road surface;
• Fines content in the wearing course material; and
• Road base properties and subgrade constituents.

Consequently, the efficacy of common dust suppressants is considered to have a limited applicability in practice in addition to negative environmental or occupational health and safety issues. Reduction in the capacity of soil water retention, prevention of infiltration of water, toxicity and detrimental effects of pH value on groundwater and soil are amongst some of the reasons traditional dust suppressants have been greatly replaced with non-traditional dust suppressants. 

Lignosulfonate is a waste product from the paper-making and timber production industry. Generally the chemical composition varies depending on the feedstock in the specific production line for the particular product in the plant. In the 1980s, lignosulfonates were known as sulphite liquor a by-product from the paper industry with the ability to create certain bonds among the particles of the mechanical soil stabilization which eventually led to dust suppression. It is considered to be environmentally benign and is water-soluble under any pH value. Essentially, it is lignin-based with molar mass ranging from 4600 to 389,000g. Its availability is in a brown powder form or a liquid form with a pH value around 4. When used as a dust suppression agent, the lignin polymers work by binding soil particles together through film formation. The chemical reaction involves; firstly a protonated H+ that dissociated from water which is followed by neutralization with the negative charges on particle surfaces leading to a reduction in the double layer thickness and form bindings. When it comes to weather conditions, the applications are only effective under dry conditions with low humidity. Lignosulfonate has low cost, non-corrosive, considered fast acting with no interruption of mining activities when used to suppress haul mine road dust, non-hygroscopic hence create a long-lasting protection, suitable for arid regions owing to good water solubility and their high organic matter content allows for good revegetation. There are also significant limitations associated with in situ material; if the wearing course contains little fine material or excessive loose gravel, these dust suppressants are not suitable and have shown poor adaptability to wet conditions, as they resolubilise and wash away – contaminating receiving waters and leaving the material untreated. They also have a reputation for becoming slippery – leading to incidents onsite. As such, although lignin is an effective dust control measure, there are limitations to its applicability.

These shortcomings grew from the temporary nature of the bond which occurred as the material that separated through evaporation is water soluble hence it washed off by precipitation and the stabilization effect failed under wet conditions. To counteract this problem, addition of lime to form an insoluble resion of calcium lignosulphonate was used. Addition of sodium or calcium bichromate was also attempted with the chromium ions would transform the lignosulphonate into a water-absorbent gel-type chemical dust and stabilisation palliative capable of considerable expansion and ability to fill up the pores of the soil. From the cellulose plant wastes from timber processing acidic raw sulphite liquor, condensed sulphite liquor and calcium oxide lignine have been utilized with success in soil stabilization and dust control. The downside to these products is related to high acidity hence tendency to be reactive particularly at high temperatures however if used in the form of a condensed and neutralized liquid it can readily be transported and used for stabilization and dust control.  

Water is still one of the most frequently employed dust suppression techniques, even in the developed world. Although water moisture prevents fugitive dust by binding fines to gravel, it must be applied frequently to maintain adequate moisture because it readily evaporates. Water agglomerates the surface particles together and in most cases it is readily available. Frequency of application depends on temperature and humidity and typically it is only effective from 1 or 2 hours but can be up to 12 hours. The hydrophilicity of water also works with limitations for hydrophobic surfaces as the surface tension between the surface is very high hence the binding ability and longevity in dust suppression is limited. Dislodgement of nanodust particles with average particle diameters smaller than the water droplets can also enhance PM2.5 in the environment if water is not made to work by superactivation and at the same time, surfaces cannot be over watered causing road instability. Consequently, although effective, the constant application of water to maintain dust suppression means that it is a labour-intensive process, has high associated equipment costs and has the potential to interrupt general traffic services and work activities. 

Salts: Calcium, sodium, and magnesium chloride have been used for dust control from earth roads since the early 1900s. The first documented application originated in 1913 when these salts were used for dust adsorption along water bound Macadam roads, which led to the conclusion that on application of salts the road stayed moist even under dry weather conditions. This behaviour was attributed to reduction of evaporation by salt and this enabling the road absorbed water from the atmosphere owing to their hygroscopic nature.  Magnesium and calcium chloride products have similar effects. They are classified as salts and in general salts improve the surface strength of sand particles through ion exchange, concentrate sand particles by reducing spacing and lead to flocculation which ultimately aids in dust suppression. Their mechanism of action involves absorption of moisture from the atmosphere and binding of material particles together in the process preventing their buoyancy caused by moving vehicles. Use of salts as dust suppressants is motivated by their deliquescent and hygroscopic nature. 

• Deliquescent is the property of dissolving and hence becoming liquid by attracting and absorbing moisture from the air. Examples are magnesium chloride and calcium chloride. 
• Hygroscopicity is the property of readily absorbing and retaining moisture. Examples are magnesium chloride and calcium chloride.

Both groups of dust suppressants depend heavily on air humidity level. MgCl2 attracts and retains moisture at a relative humidity equal to or greater than 32% (at 25°C), whilst CaCl2 needs at least 29% relative humidity (at 25°C). Studies have also shown that test sections treated with magnesium or calcium chloride solutions instead of solid salts achieve a more uniform product distribution and more efficient performance whilst also reducing the cost for annual dust control as well as environmental impact by at least 50%. The use of magnesium chloride tends to yield a harder surface than calcium chloride. Consequently, it lends itself to more applications in unsealed trafficable surfaces. Both products however, can form a slippery surface when wet, which can jeopardize traffic safety. Furthermore, they are water soluble and thus not suitable for regions with high rainfall events. Other issues with these products are high corrosivity to most metals, negative effects on road side vegetation and local water systems, and require minimum humidity to absorb moisture from air and if content of fines is high in the surface they lose abrasion and become slippery. 

Chemically, the various salts if dissolved in the pore water of the soil, supply cations which are capable of participating in the following reactions:

• exchange with the ions already present in the soil;
• adsorption over the surface of the particles 
• supply of ions connecting the soil particles for example potassium cation will be embedded between two clay sheets and connect them permanently during dry out 
• ion concentration increases, whereby the intergranular electric repulsion will be reduced 

Bitumen emulsions (or emulsified asphalt) are a mixture of bitumen, emulsifiers, surfactants and water that penetrates the soil surface. For efficient application, the asphalt is mixed into the top few centimetres of surface with a grader. The asphalt application causes emissions of volatile organic compounds (VOCs) and contributes to the creation of ground-level ozone and particulate matter, which are the main causes of smog. Additionally, these dust suppressants can contaminate waterways due to runoff and thus are rarely used nowadays. Bitumen use as a dust suppression should be avoided due to health, safety and environmental impacts that they cause.

Key to bitumen emulsions is penetration which can be achieved through dilution with water and non-volatile resins. This is very important because the water carrier enables the bitumen to penetrate to depths that enable a better bond between the granular particles of the surface. The variations in bitumen emulsion applications range from: 

• • Spray on applications that last approximately four weeks before requiring rejuvenation 
  • Thicker applications that can be blended with the soil and perform similar to sand seals which can last up to 3 years before rejuvenation. 

Bitumen emulsion performance is very much dependent on the base material and drainage. A case study conducted in USA has shown that the CSS-1 cationic, slow-setting bitumen emulsion has been very effective at dust control but require specialized equipment for application. The emulsion is diluted with water to approximately 25% solids by weight. In this case they used bitumen emulsion construction procedure which involved; 

• • The emulsion being sprayed onto the graded surface 
  • Allowed to cure for 96 hours
  • No further compaction performed on the section

In another case study conducted in the USA, a resin modified bitumen emulsion was used at a 1,36l/m2 for the first application rate then reduced down to 0,68l/m2 for the next two sprays. The dilution was 1-part emulsion to 5 parts water. The results showed that dust settled quickly and the product penetrated 50mm into the surface. One important factor to consider is the mining operations closing off a road for 96 hours could prove counter-productive to the mining operations hence one needs to consider whether the emulsion product is: 

• • Slow setting 
  • Medium setting 
  • Rapid setting 

The grading of the surface is very important because if they are too many fines for example and the bitumen is high in asphaltenes it can form a crust and fragment under the traffic and in wet weather so you need to know the grading envelope that you are working with for the application of the bitumen emulsions. In some instance, some bitumen emulsion products are difficult to maintain. Generally 1 to 2 treatments per season with higher viscosity emulsions being used for more open-graded surface materials. The follow up of the emulsion spray is always at reduced initial dosages. As mentioned before cationic slow setting emulsions are used and mixed with 5 parts of water. 

Best practice approaches: Although there are many readily available dust suppression methods on the market, there is still potential for vast improvements in the application of products and their relationship with climatic conditions, varying wearing course materials, environmental impacts, road stabilization and driving surface quality. Global Road Technology’s (GRT’s) specialists have succeeded in developing dust suppression products specialized for driving surfaces. These range from heavy industry, mining haul roads, to public roads.

As GRT’s products have a binding property, they also have the ability to contribute significantly to road stabilization. GRT products form smooth riding and non-slippery surfaces. Additionally, they have are solutions applicable across all climate types and weather conditions and in all wearing course types. GRT Managing Director Troy Adams said dust suppression products also perform well economically. 

A comparative analysis was undertaken on a 10 km stretch of road whereby traditional watering methods were compared with GRT dust suppressants. After a 3 month trial, the use of GRT products yielded a 40% cost saving over traditional water dust suppression. Additionally, the use of the GRT product also allowed for significant water conservation – up to 99.67% in this case. Furthermore, the transportation and treatment operations required for GRT dust suppression is less than that required by traditional watering, resulting in minimal operational and traffic disturbances.

For more information on Global Road Technology please contact GRT.

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REFERENCES 

• Bolander, P. Chemical Additives for Dust Control What We Have Used and What We Have Learned. Transportation Research Record 1589 Paper No.970726
• Gaspar. 1979. Chemical soil stabilization methods. Book chapter 7. Hungarian Road Research Institute
• Giummarra, G.J., Foley, G., and Cropley, S. Dust Control Australasian Experiences with Various Chemical Additives. Transportation Research Record. Paper No. 971490. 
• Jones, D. 2000. Road Dust – Just A Nuisance or a Significant Road Management Issue? South African Transport Conference. South African Transport Conference. 
• Jones, D., Kociolek, A. Surdahl, R., Bolander, P., Drewes, B., Duran, M., Fay, L., Huntington, G., James, D., Milne, C., Nahra, M., Scott, A., Vitale, B., Williams, B. 2013. Unpaved Road Dust Management: A Successful Practioner’s Handbook. Publication No. FHWA-CFL/LTD- 13-001 
• Steevens, J., Suedel, B., Gibson, A., Kennedy, A., Blackburn, W., Splichal, D., and Pierce, J.T. 2007. Pavements Research Program: Environmental Evaluation of Dust Stabilizer Products. US Army Corps of Engineers. Engineer Research and Development Center. ERDC/EL TR-07-13. 1-69.
• Xu, G., Ding, X., Kuruppu, M., Zhou, W., and Biswas, W. 2017. Research and application of non-traditional chemical stabilizers on bauxite residue (red sand) dust control, a review. Science of the Total Environment.